Digital Subscriber Line (xDSL) is a technology which allows for simultaneous voice arid data traffic to coexist over a communication channel comprising a standard telephone transmission line. Typically, a standard telephone transmission line comprises an unshielded twisted pair of copper wire having a gauge of 22-26AWG. Twisted pairs, which can be used to connect a central telephone system (a ‘central’ unit) to a subscriber's telephone (a ‘remote’ unit) can support bandwidths of up to 2 MHz through the use of digital signal processing (ASP) technology. Thus, they can be used for bandwidth-intensive applications, such as Internet access and video-on demand, as well as for carrying voice traffic. Frequency division multiplexing is used so that a puality of signals, each occupying a different frequency band, can be simultaneously sent over the same transmission line.
The voice traffic band comprises a number of frequency subbands, or channels, ranging from DC to 20 KHz The analog voice band frequency is typically specified as 200–4000 Hz Customer specified additions may include phone operation up to 8 KHz in addition to 12–16 KHz billing tones. In addition, DC to 30 Hz frequencies are typically assigned for auxiliary analog signaling purposes, such as ringing the telephone, dial pulsing and ontoff hook signaling.
ADSL data traffic bandwidth for Discrete Multitone (DMT) modulation is typically from 25 KHz–1.1 MHZ. Of this, upstream data traffic (i.e., remote unit to central unit) uses the 25 KHz–138 KHz band, while the downstream traffic (i.e., central unit to remote unit) uses the 138 KHz–1 MHZ band.
U.S. Pat. No. 5,541,955 discloses an adaptive data rate modem, The modem incorporates an adaptive data rate encoder and an adaptive data rate decoder using adaptive, parallel-branch decoding to translate received symbols into corresponding data bits. The data rate is changed automatically and dynamically without interrupting the decoding process. A constant channel symbol rate and a single signal set simplify signal acquisition and synchronization. Incoming and outgoing data are buffered, and the transmission rate is changed dynamically by a memory controller to avert buffer overflows and underflows. An optional adaptive phase-lock loop system maintains synchronization of the decoder at all data rates.
U.S. Pat. No. 5,841,840 discloses a multiple line modem and method enabling a user to automatically usurp a plurality of telephone lines for data transfer when the telephone is not in use. A user is able to switch from multiple line data operation to one (Or move) line data and one (or more) line telephone operation automatically when the telephone handset is lifted or an incoming call is detected signaling a request for voice service. The multiple line modern automatically reestablishes the data connection on the line(s) that was used for voice service when the voice service terminates. The multiple line modem allows end to end service that is transparent to the central office. When all lines are operating in data transfer mode, the aggregate data transfer rate is multiplied by the number of lines available for data transport When one line(s) is operating in voice service mode, the other line(s) maintains data transfer operation at a reduced rate. The line(s) that is used for voice service is adaptively managed by the multiple line modem to provide seamless switching between data transfer mode operation when voice service is not requested and voice service mode operation when voice service is requested.
U.S. Pat. No, 5,910,959 discloses a methodology for a modem control channel. The channel allows faster seamless rate change and precoder tap exchange than the baseline procedure for seamless rate change, allowing for more robust transmission of control information It can also be used to convey side-information in the case of multiple data applicatiors, serving the purpose of mode switching. Thus, with a single control channel both the needs for seamless rate change and transmitting control information for multiple data applications can be met.
U.S. Pat. No. 6,002,722 discloses a modern operating selectively in the voice frequency and higher frequency bands which supports multiple line codes. A DSP is used to implement different existing ADSL line codes on the same hardware platform. The modem negotiates in real time for a desired line transmission rate to accommodate line condition and service cost requirements which may be implemented at the beginning of each communication session by exchange of tones between modems. A four step MDSL modem initialization process provides line code and rate compatibility. The handshake protocol and receiver algorithm allow reliable modem synchronization over severely amplitude distorted channels and makes use of a short length sequence to train a synchronizing equalizer at the receiver. The algorithm and corresponding training sequence to train the transmitter filter are provided. After training to this sequence, a matched filter or correlator detects the inverted sync sequence. Detection of the inverted sequence signals commencement of normal reference training of the demodulation equalizers. An internal state machine in an MDSL modem records and monitors line status and notifies state change to other MDSL and host processor. The protocol for exchanging line connection management messages is a simplified LCP for MDSL. In a DMT system, a transmitter filter reduces the length of effective channel impulse response. Implementation of the filter combines time domain convolution and frequency domain multiplication to reduce needed computation power. The filter coefficients update may occur through a feedback channel.
The contents of aforementioned U.S. Pat. Nos. 5,541,955, 5,874,840, 5,910,959, and 6,002,722 are incorporated by reference to the extent necessary to understand the present invention